一、首先，看ActivityThread 的介绍：
* This manages the execution of the main thread in an
* application process, scheduling and executing activities,
* broadcasts, and other operations on it as the activity
* manager requests.大概的意思就是说：在应用程序里面管理和执行主线程，调度和执行Activity，广播以及在Activity上的其他业务操作。
二、直接上main()方法。
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}三、一句一句来解读吧。
1、Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
Trace是痕迹，踪迹的意思，是在android.os包下的一个类，主要作用是将跟踪事件写入系统跟踪缓冲区，这些跟踪事件可以用Systrace可视化工具收集和使用。看一下traceBegin（）方法，是给代码段写一个跟踪信息。
public static void traceBegin(long traceTag, String methodName) {
if (isTagEnabled(traceTag)) {
nativeTraceBegin(traceTag, methodName);
}
}
public static boolean isTagEnabled(long traceTag) {
long tags = sEnabledTags;
if (tags == TRACE_TAG_NOT_READY) {
tags = cacheEnabledTags();
}
return (tags & traceTag) != 0;
}
private static native void nativeTraceBegin(long tag, String name);这里传入了traceTag和methodName，首先就是判断跟踪标签是否启用，如果启用，就调用底层方法，开始跟踪事件。
2、SamplingProfilerIntegration.start();
这个类在com.android.internal.os包下，字面意思翻译就是“抽样分析集成工具”，是集成Dalvik抽样分析器框架。start()是静态方法：
/**
* Starts the profiler if profiling is enabled.
*/
public static void start() {
if (!enabled) {
return;
}
if (samplingProfiler != null) {
Log.e(TAG, "SamplingProfilerIntegration already started at " + new Date(startMillis));
return;
}
ThreadGroup group = Thread.currentThread().getThreadGroup();
SamplingProfiler.ThreadSet threadSet = SamplingProfiler.newThreadGroupThreadSet(group);
samplingProfiler = new SamplingProfiler(samplingProfilerDepth, threadSet);
samplingProfiler.start(samplingProfilerMilliseconds);
startMillis = System.currentTimeMillis();
}首先拿到当前线程的线程组group ，threadSet ，然后开启抽样分析。
3、CloseGuard.setEnabled(false);
CloseGuard是dalvik.system包下的一个类。CloseGuard 是一种资源清理机制，资源应该被显式关闭清理。setEnabled（）明显就是使其失效，不过会在debug builds的时候重现变得有效。
* CloseGuard is a mechanism for flagging implicit finalizer cleanup of
* resources that should have been cleaned up by explicit close
* methods (aka "explicit termination methods" in Effective Java).4、Environment.initForCurrentUser();
Environment大家都很熟悉：环境。
public static void initForCurrentUser() {
final int userId = UserHandle.myUserId();
sCurrentUser = new UserEnvironment(userId);
}首先拿到当前进程的Id，然后初始化一个UserEnvironment实例。
5、EventLogger.setReporter(new EventLoggingReporter());
看源码说明，Set the reporter for event logging in libcore，简单就是说给lib库设置事件打印记者。
6、final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
确保可信证书存储在正确的地方以便于查找CA证书。
7、Process.setArgV0("<pre-initialized>");
Process是管理系统流程的工具。setArgV0（）是一个本地方法：
/**
* Change this process's argv[0] parameter. This can be useful to show
* more descriptive information in things like the 'ps' command.
*
* @param text The new name of this process.
*
* {@hide}
*/
public static final native void setArgV0(String text);8、Looper.prepareMainLooper();
终于开始干正事了，这里初始化了主线程。首先是在prepare（）里面判断当前线程是否为空，因为Looper只允许有一个主线程。
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}9、ActivityThread thread = new ActivityThread();
thread.attach(false);
这里直接实例化了一个ActivityThread 对象，并调用了attach(false)。
private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
ViewRootImpl.addFirstDrawHandler(new Runnable() {
@Override
public void run() {
ensureJitEnabled();
}
});
android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
UserHandle.myUserId());
RuntimeInit.setApplicationObject(mAppThread.asBinder());
final IActivityManager mgr = ActivityManagerNative.getDefault();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
// Watch for getting close to heap limit.
BinderInternal.addGcWatcher(new Runnable() {
@Override public void run() {
if (!mSomeActivitiesChanged) {
return;
}
Runtime runtime = Runtime.getRuntime();
long dalvikMax = runtime.maxMemory();
long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
if (dalvikUsed > ((3*dalvikMax)/4)) {
if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
+ " total=" + (runtime.totalMemory()/1024)
+ " used=" + (dalvikUsed/1024));
mSomeActivitiesChanged = false;
try {
mgr.releaseSomeActivities(mAppThread);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
}
});
} else {
// Don't set application object here -- if the system crashes,
// we can't display an alert, we just want to die die die.
android.ddm.DdmHandleAppName.setAppName("system_process",
UserHandle.myUserId());
try {
mInstrumentation = new Instrumentation();
ContextImpl context = ContextImpl.createAppContext(
this, getSystemContext().mPackageInfo);
mInitialApplication = context.mPackageInfo.makeApplication(true, null);
mInitialApplication.onCreate();
} catch (Exception e) {
throw new RuntimeException(
"Unable to instantiate Application():" + e.toString(), e);
}
}
// add dropbox logging to libcore
DropBox.setReporter(new DropBoxReporter());
ViewRootImpl.addConfigCallback(new ComponentCallbacks2() {
@Override
public void onConfigurationChanged(Configuration newConfig) {
synchronized (mResourcesManager) {
// We need to apply this change to the resources
// immediately, because upon returning the view
// hierarchy will be informed about it.
if (mResourcesManager.applyConfigurationToResourcesLocked(newConfig, null)) {
updateLocaleListFromAppContext(mInitialApplication.getApplicationContext(),
mResourcesManager.getConfiguration().getLocales());
// This actually changed the resources! Tell
// everyone about it.
if (mPendingConfiguration == null ||
mPendingConfiguration.isOtherSeqNewer(newConfig)) {
mPendingConfiguration = newConfig;
sendMessage(H.CONFIGURATION_CHANGED, newConfig);
}
}
}
}
@Override
public void onLowMemory() {
}
@Override
public void onTrimMemory(int level) {
}
});
}有耐心的就看看。
10、if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
判断当前主线程的Handler是否为空。
11、if (false) {
Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread"));
}
12、Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
跟前面的Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");相呼应，结束跟踪。
13、Looper.loop();
最重要的一句来了，前面Looper.prepareMainLooper()开启主线程，而这里的loop()就是处理主线程程序的关键段，所有通过Handler传到主线程的Message都会缓存在Looper的消息队列Queue里面，loop()里有个for(;;)循环，不断的从消息队列里面拿出消息进行处理，然后就有了事件的信息交流啦。
14、最后一句throw new RuntimeException("Main thread loop unexpectedly exited");
必要的异常处理语句。
最后说一句，若有任何问题和疑问，欢迎指正。
